Which of the following is NOT one of the eight kingdoms of organisms?

The classification of living organisms has expanded beyond the traditional five kingdoms to include eight kingdoms, which are based on genetic and evolutionary relationships. In this system, organisms are classified into various categories, including protists, plants, fungi, and animals.

Among the options presented, mammals are not classified as a separate kingdom. Instead, mammals belong to the kingdom Animalia, which encompasses a vast range of multicellular organisms that share certain characteristics, such as being heterotrophic and having complex cellular structures. The distinction of mammals within their own category is more specific, dealing with their subclassification within Animalia, rather than representing a separate kingdom.

In contrast, protists, gymnosperms, and eubacteria fit into the classification of the eight kingdoms. Protists form a diverse group including organisms that do not fit into the other kingdoms, gymnosperms represent a group of seed-producing plants, and eubacteria consist of true bacteria that are characterized by their prokaryotic cells. Therefore, mammals being part of the Animalia kingdom makes them not part of the eight kingdoms in this classification context.